A chicken fetal antigen (CFA) localized to the red blood cell membrane has been found to disappear with chicken development and to reappear in adult AMV-induced leukemic chickens. An understanding of the genetic regulatory mechanism controlling the reappearance of the CFA is being investigated. A differentiation blockage or cell maturation blockage hypothesis has been postulated as a possible explanation for the reappearance of CFA in adult leukemic chickens. Additional membrane and intracellular markers will be developed in order to further test the cell maturation blockage hypothesis. Immunological and serological analyses will be utilized to detect additional cellular membrane antigens common to fetal and leukemic chickens. In addition to the above immunological techniques, electrophoretic techniques will be utilized to assay for intracellular components of individual cells. AMV-induced leukemic adult chickens will be examined for the presence or absence of these fetal-adult markers. Immune tolerance studies will be made in order to examine the inability of the immune system to control the leukemic state. The adverse effects of immunotherapy will be examined.